With conventional memory cell structures approaching their scaling limit, many types of memory cell structures have attracted extensive research. For example, variable resistive element type memory cells, such as magnetic random access memory (MRAM), resistive random access memory (RRAM), and the like have continued to gain popularity. In variable resistive memory cells, the difference in storage state appears as the difference between threshold voltages of the memory transistor. Among resistive memory, RRAM is a good candidate for use in low-scale applications due to a low operating voltage, fast access time, and good endurance.
The basic component of an RRAM cell is a variable resistor. The variable resistor can be programmed to have high resistance or low resistance (in two-state memory circuits), or any intermediate resistance value (in multi-state memory circuits). The different resistance values of the RRAM cell represent the information stored in the RRAM circuit. The advantages of RRAM are the simplicity of the circuit (leading to smaller devices), the non-volatile characteristic of the resistor memory cell, and the stability of the memory state.
Since a resistor is a passive component and cannot be actively influence nearby electrical components, a basic RRAM cell can be just a variable resistor, arranged in a cross point resistor network to form a cross point memory array. To prevent cross talk or parasitic current paths, a RRAM cell can further include a diode, and this combination is sometimes called a 1R1D (or 1D1R) cross point memory cell. To provide better access, an RRAM cell can include an access transistor, and this combination is sometimes called a 1R1T (or 1T1R) cross point memory cell.
The resistance state of an RRAM cell is referred to the storing (writing) or sensing (reading) methodology of the RRAM circuit. The term resistance state is related to the resistance value of the memory resistor (the resistance state can then be said to be the resistance of the memory resistor), but sensing the resistance value of the memory resistor often means sensing the voltage across the memory resistor (the resistance state can then be said to be the voltage across the memory resistor), or sensing the current through the memory resistor (the resistance state then can be said to be the current through the memory resistor).
In conventional manufacturing processes for RRAM cells, a dielectric is sandwiched between a top electrode and a bottom electrode. One example of a structural state for an RRAM cell is a chalcogenide alloy serving as the dielectric material. Such RRAM cells are typically called phase change memory (PCM) cells. Chalcogenide alloys can exhibit two different stable reversible structural states, namely an amorphous state with high electrical resistance and a polycrystalline state with lower electrical resistance. Resistive heating by an electrical current can be used to change the phase of the chalcogenide materials.
Other RRAM structures include the use of a (manganite) Colossal magnetoresistive (CMR) material as the dielectric material sandwiched between the electrodes. Other types of RRAM devices include electric-pulse-induced resistance (EPIR) devices. The EPIR effect encompasses the reversible change of resistance of a thin-oxide film, such as Pr1−x Cax MnO3 (PCMO), under the application of short, low voltage pulses. The information can be stored (or written) to such an RRAM device by applying the voltage pulses to the CMR material. The information can then be sensed (or read) by sensing the resistance across the CMR material using a constant current source, or by sensing the current through the CMR material using a constant voltage source. However, the typical structure of such a dielectric material sandwiched between two electrodes often results in the resist ratio of conventional RRAM cells (Ron/Roff) to be relatively low and is often difficult to sense in certain applications. Accordingly, an increased resistance ratio for these type of RRAM cells is desired.